Field of the Invention
The present invention relates to an electrode for a secondary battery, and a secondary battery using the same, and more particularly, to a cathode electrode for a nonaqueous electrolyte secondary battery capable of improving cycle characteristics.
Description of the Related Art
Recently, as a secondary battery capable of being repetitively charged and discharged, a lithium (Li) ion secondary battery has attracted attention for a reduction in the amount of petroleum used or a greenhouse effect gas, and additional diversification or optimization of energy infrastructures. Particularly, development of usage in an electric vehicle, a hybrid electric vehicle, and a fuel battery vehicle has been expected. In the electric vehicle, an improvement in travel distance is demanded, and an improvement in energy density of a secondary battery will be further demanded in the future.
As to a cathode electrode of the lithium ion secondary battery, a graphite electrode is typically used. A theoretical capacity of graphite is 372 mAhg (active material)−1. In contrast, as an active material that exhibits a capacity greater than that of graphite, recently, silicon (Si) and tin (Sn) have attracted attention. A theoretical capacity of Si is 4200 mAhg (active material)−1, and a theoretical capacity of Sn is 990 mAhg (active material)−1. On the other hand, Si has a capacity that is approximately 11 times that of graphite, and thus a volume change in accordance with lithiation and delithiation of Li also increases. The volume increases approximately 4 times due to lithiation of Li.
In the electrode using the active material having a large capacity in comparison to graphite, due to a large volume change in accordance with charge and discharge, there are concerns such as a disconnection in a conduction path of the electrode, detachment from the electrode in accordance with pulverization, separation of a current collector and an active material layer, and the like. These may become a cause of deterioration of the cycle characteristics of a secondary battery.
In addition, as a main cause of deterioration of cycle characteristics, consumption of Li in accordance with formation of a solid electrolyte interface (SEI) (a film that is generated in a case of using ethylene carbonate, and inactivates and stabilizes a surface of an active material so as to insert lithium therethrough) can be exemplified. Particularly, in a case of using a Si-based active material, in which a large volume variation in accordance with charge and discharge occurs, as an electrode, consumption of Li in accordance with formation of SEI is also negligible when considering that breakage and generation of SEI may be repeated.
For example, Japanese Unexamined Patent Application, First Publication No. H11-354104 discloses an invention relating to formation of a stable SEI. Here, an electrode is processed with a silane coupling agent and the like so as to have LUMO of 1.0 eV or greater (LUMO; Lowest Unoccupied Molecular Orbital; a molecular orbital that is not occupied with electrons and has the lowest energy, the lowest unoccupied orbital). In Japanese Unexamined Patent Application, First Publication No. H11-354104, a stable SEI is formed, but there is a concern that an increase in electrode interfacial resistance is also caused, and thus a decrease in capacity may occur.
In addition, for example, Japanese Unexamined Patent Application, First Publication No. 2011-49046 describes that a binder and an active material are covalently bonded to each other as means for preventing the active material from being detached from an electrode. According to this, it is possible to suppress deterioration of the cycle characteristics. However, if the coupling is cut out once, the covalent coupling between the binder and the active material is not recovered. Accordingly, although it can be said that an improvement in characteristics is exhibited, an additional improvement in the cycle characteristics is demanded.
In addition, for example, PCT International Publication No. WO 2011/140150 describes that sodium alginate is used as the binder. The active material and the binder are bound to each other due to an electrostatic mutual operation, and coupling is maintained in a self-recovery manner. Accordingly, it is considered that the cycle characteristics can be improved.
However, in the inventions described in Japanese Unexamined Patent Application, First Publication No. H11-354104 to PCT International Publication No. WO 2011/140150, an effect of improving the cycle characteristics of the secondary battery is low, and thus an additional improvement in the cycle characteristics is demanded.
The invention has been made in consideration of the above-described problems, and an object thereof is to provide an electrode for a secondary battery capable of improving cycle characteristics, and a secondary battery.